Bandgap reference voltage generator circuits are well known in the art. Such circuits are configured to generate a reference voltage that is approximately equal to the bandgap voltage (Vbg) of silicon (i.e., 1.205 Volts at zero degrees Kelvin). Generating such a voltage from a power supply voltage in excess of 1.8 Volts, for example, is of no concern. However, now integrated circuit devices are provided with supply voltages well below 1.8 Volts. Indeed, some integrated circuit devices or circuit portions within the integrated circuit device may be powered with an input supply voltage as low as 0.5 Volts. Operating analog circuitry, such as bandgap reference voltage generator circuits, at such low input supply voltage levels is a challenge.
It is further recognized in the art that the reference voltage needed may be less than the bandgap voltage (i.e., a sub-bandgap voltage) and in particular may be an integer fraction of the bandgap voltage. For example, for analog circuits operating at low supply voltages, the reference voltage must be lower than the supply voltage. An analog circuit operating with a low on-chip supply voltage of 1.0 Volts, for example, may require a reference voltage of 0.6 Volts, which can be obtained as an integer fraction (1.205/2) of the bandgap voltage.
An example of a fractional bandgap reference voltage generator circuit is the so-called Banba bandgap reference voltage generator circuit 10 as shown in FIG. 1. See also, Banba, et al. “A CMOS Bandgap Reference Circuit with Sub-1-V Operation,” IEEE Journal of Solid State Circuits, vol. 34, pp. 670-674, May 1999. The emitter area of transistor Q1 is n times larger than the emitter area of transistor Q2. In a common configuration, n=8. Both transistor Q1 and transistor Q2 are configured as diode-connected devices. The operational amplifier drives the gates of transistors M1 and M2 to force the voltage at the inverting input of the operational amplifier to equal the voltage at the non-inverting input of the operational amplifier. With these voltages being equal, the current I2 in the resistor R2 is proportional to the base to emitter voltage (Vbe) of transistor Q2 (i.e., I2=Vbe/R2). The current I1 flowing through each of the transistors Q1 and Q2 is given by I1=VT ln(n)/R1. As a result, the current Im flowing through each of the transistors M1 and M2 is Im=(VT ln(n)/R1)+(Vbe/R2). The first component of the current Im is proportional to absolute temperature (PTAT) and the second component is complementary to absolute temperature (CTAT). Thus, the current Im can be made temperature independent (i.e., having an at or near zero temperature coefficient). This current Im is mirrored using a current mirror circuit formed by transistor M3 to generate a temperature independent output current Io. The output current Io flows through resistor R3 to develop the output reference voltage Vref (where Vref=(R3/R2)(VT(R2/R1)ln(n)+Vbe.) If R3=R2/N, then a fractional bandgap reference voltage Vref=Vbg/N is generated. More specifically, the ratio of resistances for R2/R1 is chosen so that the slope of the PTAT voltage with temperature cancels the slope of the CTAT voltage Vbe with temperature. Generally, R2/R1 is approximately equal to 9-10 if n=8 in order to balance the slopes and obtain a compensated voltage. Mathematically, this may be represented as: R2*log(n)/R1=−(dVbe/dT)/(dVT/dT) where d/dT is the derivative with respect to temperature.
For low power applications, it is important for the currents in the reference voltage generator circuit 10 to be small. This necessitates the use of large resistance value resistors which occupy a correspondingly large amount of integrated circuit chip area. There is accordingly a need in the art for a fractional bandgap reference voltage generator circuit that supports low power supply operation with low current (i.e., low power operation) and a reduced occupation of integrated circuit area.